The only restriction that Elgg places on a password is that it must be at least 6 characters long by default, though this may be changed in /elgg-config/settings.php. Additional criteria can be added by a plugin by registering for the registeruser:validate:password plugin hook.

Passwords are never stored, only salted hashes produced with bcrypt. This is done via the standard password_hash() function. On older systems, the password-compat polyfill is used, but the algorithm is identical.

Elgg installations created before version 1.10 may have residual “legacy” password hashes created using salted MD5. These are migrated to bcrypt as users log in, and will be completely removed when a system is upgraded to Elgg 3.0. In the meantime we’re happy to assist site owners to manually remove these legacy hashes, though it would force those users to reset their passwords.

If a user forgets his password, a new random password can be requested. After the request, an email is sent with a unique URL. When the user visits that URL, a new random password is sent to the user through email.

Elgg uses PHP’s session handling with custom handlers. Session data is stored in the database. The session cookie contains the session id that links the user to the browser. The user’s metadata is stored in the session including GUID, username, email address. The session’s lifetime is controlled through the server’s PHP configuration.

Besides protecting against session fixation attacks, Elgg also has a further check to try to defeat session hijacking if the session identifier is compromised. Elgg stores a hash of the browser’s user agent and a site secret as a session fingerprint. The use of the site secret is rather superfluous but checking the user agent might prevent some session hijacking attempts.

To allow users to stay logged in for a longer period of time regardless of whether the browser has been closed, Elgg uses a cookie (called elggperm) that contains what could be considered a super session identifier. This identifier is stored in a cookies table. When a session is being initiated, Elgg checks for the presence of the elggperm cookie. If it exists and the session code in the cookie matches the code in the cookies table, the corresponding user is automatically logged in.

To replace Elgg’s default user authentication system, a plugin would have to replace the default action with its own through register_action(). It would also have to register its own pam handler using register_pam_handler().

Note

The pam_authenticate() function used to call the different modules has a bug related to the importance variable.

Filtering is used in Elgg to make XSS attacks more difficult. The purpose of the filtering is to remove Javascript and other dangerous input from users.

Filtering is performed through the function filter_tags(). This function takes in a string and returns a filtered string. It triggers a validate,input plugin hook.

By default Elgg comes with the htmLawed filtering code as a plugin. Developers can drop in any additional or replacement filtering code as a plugin.

The filter_tags() function is called on any user input as long as the input is obtained through a call to get_input(). If for some reason a developer did not want to perform the default filtering on some user input, the get_input() function has a parameter for turning off filtering.

Elgg generates security tokens to prevent cross-site request forgery. These are embedded in all forms and state-modifying AJAX requests as long as the correct API is used. Read more in the Forms + Actions developer guide.